1. Field of the Invention
The invention pertains to apparatus and methods for detecting leaks in gas-tight components. More particularly, the invention pertains to apparatus and methods for detecting leaks using tracer gases to measure leakage into or out of a pressurized or evacuated component.
2. Description of Related Art
In the field of gas-handling equipment, for example, refrigeration components, such as heat exchangers and the like, there is typically the requirement that every system is tested before shipment to ensure that no refrigerant leakage is occurring. The 100% test requirement often represents the critical path or bottleneck in production of items such as refrigerators and air conditioners. Testing may be done in any of several ways, which will be described in further detail:
A partially finished component (e.g., a heat exchanger) may be pressurized with refrigerant (e.g., R410A) or tracer gas (e.g., helium) and placed into a chamber that is held at a partial vacuum. A gas sampling system extracts gas from the chamber and analyzes this gas for traces of the refrigerant or tracer gas, using various conventional detection means, such as IR absorption, mass spectroscopy, etc. There are several engineering tradeoffs that exist in designing and using such systems. First, the size of the chamber largely determines how fast the system can be evacuated to the desired test pressure, and the lower the desired test pressure, the longer the pump down cycle; alternatively, a larger vacuum pump can be used but this adds cost to the system. Second, if one uses a somewhat higher pressure in the chamber, there is more gas competing with the tracer gas and therefore it takes longer for leaking tracer gas to diffuse from the point of the leak to the detector; for this reason, such systems occasionally employ mixers or fans within the test chamber to enhance the outward diffusion of the leaking tracer gas.
Alternatively, atmospheric detectors or “sniffers” are available to test a pressurized component without using a chamber. In this situation, the operator holds a wand and manually “sniffs” each braze joint, connection, or other potential source of leaks. This is very time-consuming, if done properly, but is most often used for checking an entire finished system such as a complete refrigerator that has relatively few possible leak points (assuming the heat exchanger has already been leak tested separately before assembly) and is too big to be practically tested in a vacuum chamber.
Traditional atmospheric accumulation is employed to test a pressurized component with the use of an enclosure at atmospheric conditions. This method is used to test the entire system. When the necessary dwell time has elapsed in order to allow the tracer gas concentration to increase, with or without mechanical assistance, the air and tracer gas mix is sampled with a detector. The cycle time is constrained by detection limits, pressure, and volume considerations. The transport of the tracer gas molecules to the detector is limited by diffusion through the other gas molecules at atmospheric pressure. In addition, the gas molecules at atmospheric pressure dilute the tracer gas molecules. The dwell time necessary to accumulate sufficient tracer gas for reliable go/no go test may be unacceptable in a production line setting.